We have continued our studies on the biology of the hypothalamic releasing factor thyrotropin releasing hormone. We have isolated and characterized an enzyme from procine brain extracts that has an imidopeptidase action on histidylprolineamide. The isolated enzyme is inhibited noncompetitively by a variety of polypeptide hormones, suggesting that the metabolism of thyroptropin releasing of thyrotropin releasing hormone may be regulated by pituitary hormones. We have accumulated increasing evidence that a metabolite of thyrotropin releasing hormone, histidyl-roline diketopiperazine, is a biologically active compound. In addition to antagonizing ethanol-induced sleep in rats, the diketopiperazine produces hypothermia in rats and transiently elevates the level of cGMP in rat brain. We conclude that thyrotropin releasing hormone is an active principle in its own right, but that the enzymatic conversion to histodyl-proline diketopiperazine plays an important role in modulating the effects of the hormone.